Ok, found it. It sits in shader inputs, here is the shader that works:

float4x4 Projection;

Texture2D FontTexture;
sampler TextureSampler = sampler_state 
{ 
    texture = <FontTexture>; 
    magfilter = LINEAR; 
    minfilter = LINEAR; 
    mipfilter = LINEAR; 
    AddressU = clamp; 
    AddressV = clamp; 
};

struct VertexShaderInput
{
    float4 Position : POSITION0;
    float2 Texture : TEXCOORD0;
    float4 Color : COLOR0;
};

struct VertexShaderOutput
{
    float4 Position : POSITION0;
    float2 Texture : TEXCOORD0;
    float4 Color : COLOR0;
};

VertexShaderOutput VertexShaderFunction(VertexShaderInput input)
{
    VertexShaderOutput output;

    output.Texture = input.Texture;
    output.Position = mul(input.Position, Projection);
    output.Color = input.Color;

    return output;
}

float4 PixelShaderFunction(VertexShaderOutput input) : COLOR0
{
    float4 originalColor = tex2D(TextureSampler, input.Texture);
    return originalColor*input.Color;
}

technique Technique1
{
    pass Pass1
    {
        VertexShader = compile vs_2_0 VertexShaderFunction();
        PixelShader = compile ps_2_0 PixelShaderFunction();
    }
}

Ok, found it. It sits in shader inputs, here is the shader that works:

float4x4 Projection;

Texture2D FontTexture;
sampler TextureSampler = sampler_state 
{ 
    texture = <FontTexture>; 
    magfilter = LINEAR; 
    minfilter = LINEAR; 
    mipfilter = LINEAR; 
    AddressU = clamp; 
    AddressV = clamp; 
};

struct VertexShaderInput
{
    float4 Position : POSITION0;
    float2 Texture : TEXCOORD0;
    float4 Color : COLOR0;
};

struct VertexShaderOutput
{
    float4 Position : POSITION0;
    float2 Texture : TEXCOORD0;
    float4 Color : COLOR0;
};

VertexShaderOutput VertexShaderFunction(VertexShaderInput input)
{
    VertexShaderOutput output;

    output.Texture = input.Texture;
    output.Position = mul(input.Position, Projection);
    output.Color = input.Color;

    return output;
}

float4 PixelShaderFunction(VertexShaderOutput input) : COLOR0
{
    float4 originalColor = tex2D(TextureSampler, input.Texture);
    return originalColor*input.Color;
}

technique Technique1
{
    pass Pass1
    {
        VertexShader = compile vs_2_0 VertexShaderFunction();
        PixelShader = compile ps_2_0 PixelShaderFunction();
    }
}

UPDATE: as I subsequently learned, the shader can be further simplified like this:

Texture2D FontTexture;
sampler TextureSampler = sampler_state 
{ 
    texture = <FontTexture>; 
    magfilter = LINEAR; 
    minfilter = LINEAR; 
    mipfilter = LINEAR; 
    AddressU = clamp; 
    AddressV = clamp; 
};

float4 main(float4 color : COLOR0, float2 texCoord : TEXCOORD0) : COLOR0
{
    float4 originalColor = tex2D(TextureSampler, texCoord);
    return originalColor*color;
}

technique Technique1
{
    pass Pass1
    {
        PixelShader = compile ps_2_0 main();
    }
}

See Custom SpriteEffects Sample for further info.